1. Field of the Invention
The present invention relates to an electric resistance welding method for welding a workpiece which is made up by laminating a plurality of sheets of metal of different thicknesses, the welding being made by means of a spot welding gun having a pair of electrode tips.
2. Description of the Related Art
A spot welding gun is arranged such that a workpiece is pinched between a pair of electrode tips and is pressed (i.e., an electrode force is applied) and, in this state, the electrode tips are charged with electricity to thereby weld the workpiece by electric resistance welding. Conventionally, an equalizing mechanism is built in the spot welding gun in order to substantially equalize the electrode forces of both electrodes to the workpiece.
When a workpiece W which is made up by laminating a thin sheet of metal (also called a thin plate) b on top of two thick sheets of metal (also called thick plates) "a", "a" as shown in FIGS. 6A, 6B is subjected to electric resistance welding, if an electrode force of each of the electrode tips T to the workpiece W is made even, there will be formed a nugget N (i.e., a molten portion due to heat generation by resistance during electric charging) of the same size between the thick plates "a", "a" and between the upper thick plate "a" and the thin plate b. As shown in FIG. 6A, if the nugget N is formed in such a size as to be sufficient to secure a weld strength between the thick plates "a", "a", the nugget N on the side of the thin plate b will reach the surface of the thin plate b. This will cause the occurrence of spatters and/or adhesion of the electrode tip to the workpiece through melting. Therefore, it becomes necessary to decrease the amount of electric charging to thereby minimize the nugget N to suit the thin plate b, as shown in FIG. 6B. As a consequence, it becomes difficult to secure a weld strength between the thick plates "a", "a".
In view of the above-described point, the present invention has an object of providing an electric resistance welding method in which the occurrence of spatters and/or adhesion of the electrode tip to the workpiece through melting is prevented and in which the weld strength is improved.